The present invention relates to ramp generating circuits and more particularly to ramp generating circuits having compensation means to automatically adjust for operational variations in circuit components.
Circuits for generating ramp function outputs are well known in the art. Likewise, ramp function generating circuits employing means for compensating for changes in the system parameters are well known. In particular, such circuits have been adapted to provide the sweep generation voltage in cathode ray tubes employed in automobile ignition analysis apparatus. In such apparatus, it is common to have a cathode ray tube having a calibrated grid on the face thereof. The sweep across the face of the cathode ray tube displays one or more ignition pulses against the calibrated display. As the speed of revolution of the engine increases or decreases, the time interval between adjacent ignition pulses changes. Unless provision is made for changing the slope of the ramp function generator driving the sweep of the cathode ray tube, the sweep duration vis-a-vis the calibration markings on the face of the cathode ray tube will no longer be in synchronization. Accordingly, in the prior art, the time of occurrence of the ignition pulses is sensed along with the attaining of the end of the sweep voltage. Either continuous or momentary adjustments are applied to the ramp function generator in order to increase or decrease the slope in a manner which will achieve the desired sweep and ignition pulse coincidence. The problem in such apparatus is one of attaining a fairly rapid response with a relatively low degree of accuracy required. To accomplish these ends, the prior art employs a technique wherein a proportional adjustment is made to the slope of the ramp function generator. That is, the greater the deviation from coincidence between the occurrence of the spark pulse from the ignition and the end of the sweep, the greater the change applied to the slope. Likewise, a bidirectional adjustment is made wherein the slope of the ramp function generator is driven either up or down to compensate for sweeps which are taking too long as well as sweeps which are occurring too rapidly, respectively.
In certain applications, the time of response to changes is not critical, whereas the accuracy required is of paramount importance. In particular, in analog-to-digital conversion, digital voltmeter circuits, and other apparatus to be discussed in greater detail hereinafter, a high precision ramp function can be put to good advantage. In such circuits, the problem is one of component tolerance drift. As the circuit components age and are subjected to temperature extremes, etc., the calibration of the system may drift causing the required accuracy to be lost. Typically, such inaccuracies can develop over a period of minutes, or even hours, days and weeks. In the prior art discussed above, the proportional adjustments made to the slope of the ramp function being generated are made to compensate for changes in the time base of reference (spark plug firings) since component drift is considered negligible in the particular application. In the present case, however, only time, as represented by a clock frequency generator, can be counted on as being an accurate base from which to establish and compensate for variances in the system components. That is, in prior art applications the time base employed varied whereas, in the application of the present invention, the time base is substantially fixed.
Wherefore, it is the object of the present invention to provide an improved circuit for generating a precision ramp function wherein an oscillator is used to establish a time base which is then used as a known reference point from which to establish a correction factor to the slope of the generated ramp function to compensate for drift in the components of the circuit.